In a medium saucepan cook 1/2 cup onion and garlic in hot oil until tender but not brown. Carefully stir in the drained beans, undrained tomatoes, and ground red pepper. Bring to a boil; reduce heat. Simmer, uncovered for 15 minutes.
To serve, mound rice on individual plates; make a well in the center. Spoon black bean mixture into centers. Sprinkle with chopped onion, if desired.